Ok, maybe instead of saying "thermos," I should have said, vacuum flask or dewar flask.

Wikipedia says, "Thermos, their tradename for their flasks, remains a registered trademark in some countries but was declared a genericized trademark in the U.S. in 1963 as it is colloquially synonymous with vacuum flasks in general. In fact, it is far more common for Americans to use the term thermos than vacuum flask." Since I'm an American, I hope you'll forgive me for using "thermos," when I meant vacuum flask.

That first link points to a foam insulated container, which is made under the Thermos brand label, but is not a vacuum flask (it uses foam insulation). Perhaps more importantly, it doesn't have a microwave in it... it is merely microwave safe.

None of the other links even indicate how they're insulated.

Naturally, any stainless steel vacuum thermos, or metallized glass vacuum thermos, cannot be put inside of a microwave (or at least, not with useful results).

However, if the microwave emmiter is inside of the flask, the reflective metal (or metallized glass) interior should be perfect for ensuring that any microwaves which pass through the top region of the food, will reflect in a way that gives them another chance to reach the food.